1. Field of the Invention
The present invention relates to a magnetoresistive effect element having an arrangement for obtaining a magnetoresistive change by causing a current to flow in the direction perpendicular to the layer surface and a magnetic memory device including this magnetoresistive effect element.
2. Description of the Related Art
As information communication devices, in particular, personal small information communication devices such as portable terminal devices (e.g. personal digital assistants) are widely spreading, it is requested that devices such as memories and logic devices comprising these information communication devices or portable terminal devices should become higher in performance, such as they should become higher in integration degree, they can operate at higher speed and they can consume lesser electric power. Particularly, technologies that can make nonvolatile memories become higher in density and larger in storage capacity are becoming more and more important as complementary technologies for replacing hard disk devices and optical disk devices with nonvolatile memories because it is essentially difficult to miniaturize hard disk devices and optical disk devices because they have their movable portions (e.g. head seek mechanism and head rotation mechanism).
Flash memories using semiconductors, an FRAM (ferro electric random-access memory) using a ferro dielectric material and so on are known as nonvolatile memories.
However, flash memories have a defect that the information writing speed thereof is slow as the order of microseconds. On the other hand, it has been pointed out that the FRAM cannot be rewritten so many times.
A magnetic memory device called an MRAM (magnetic random-access memory) device that had been described in xe2x80x9cWang et al., IEEE Trans. Magn. 33 (1997), 4498xe2x80x9d, receives a remarkable attention as a nonvolatile memory that can overcome these defects. This MRAM is simple in structure and therefore can be easily integrated at high integration degree and since the MRAM is able to record by rotation of magnetic moment, it can be rewritten a large number of times. Further, it is expected that the MRAM has very high access time and it has already been confirmed that the MRAM can be operated at speed in the order of nanoseconds.
A magnetoresistive effect element used in this MRAM and especially ferromagnetic tunnel junction (MTJ (magnetic tunnel junction)) is essentially composed of a laminated layer construction of a ferromagnetic material layer, a tunnel barrier layer and a ferromagnetic material layer. In this element, when an external magnetic field is applied to the ferromagnetic material layers under the condition in which a constant current flows through the ferromagnetic material layers, magnetoresistive effect appears in response to a relative angle of the magnetizations of the two ferromagnetic material layers. When the magnetization directions of the two ferromagnetic material layers are anti-parallel, a resistance value becomes the maximum. When the magnetization directions of the two ferromagnetic material layers are parallel to each other, a resistance value becomes the minimum. Functions of the memory element can be achieved when anti-parallel and parallel states are produced with application of external magnetic fields.
In particular, in a spin-valve type TMR element, one ferromagnetic material layer is coupled to an adjacent antiferromagnetic material layer in an antiferromagnetic fashion and thereby formed as a magnetization fixed layer whose magnetization direction is constantly made constant. The other ferromagnetic material layer is formed as a magnetization free layer whose magnetization direction is easily inverted with application of external magnetic fields and the like. Then, this magnetization free layer is formed as an information recording layer in a magnetic memory.
For the TMR element of a spin-valve structure, a changing ratio of a resistance value in the TMR element is expressed as the following equation (A) where P1, P2 represent spin polarizabilities of the respective ferromagnetic material layers:
2P1P2/(1xe2x88x92P1P2)xe2x80x83xe2x80x83(A)
Accordingly, the changing ratio of the resistance value increases as the respective spin polarizabilities increase. With respect to a relationship between materials for use with the ferromagnetic material layers and this resistance change ratio, ferromagnetic elements of Fe group, such as Fe, Co, Ni and alloys of these three kinds of elements have been reported so far.
Fundamentally, the MRAM comprises a plurality of bit write lines (so-called bit lines), a plurality of word write lines (so-called word lines) and TMR elements disposed at intersection points between these bit write lines and word write lines as magnetic memory elements as shown in Japanese laid-open patent application No. 10-116490, for example. When information is to be written in such MRAM, information is selectively written in the TMR elements by using asteroid characteristics.
Bit write lines and word write lines for use with the MRAM are made of Cu or Al conductive thin films which are generally used in semiconductors. When information is written in the element by a write line having a width of 0.25 xcexcm with application of inverted magnetic fields, for example, a current of approximately 2 mA was required. When the thickness of the write line is the same as the line width, a current density obtained at that time reaches 3.2xc3x97106 A/cm2 and which is a limit value in breaking wires due to electro-migration. From a problem of heat generated by a write current and from a standpoint for decreasing power consumption, it is necessary to decrease this write current.
As a method for realizing decrease of the write current in the MRAM, there may be enumerated a method for decreasing coercive force of the TMR element. The coercive force of the TMR element is properly determined based upon suitable factors such as size and shape of element, film arrangement and selection of materials.
However, when the TMR element is microminiaturized in order to increase the recording density of the MRAM, for example, there arises a disadvantage in which the coercive force of the TMR element increases.
Accordingly, in order to microminiaturize the MRAM (to integrate the MRAM with high integration degree) and to decrease the write current at the same time, the coercive force of the TMR element should be decreased from a standpoint. of materials.
In the MRAM, when magnetic characteristics of the TMR elements fluctuate at every element or magnetic characteristics fluctuate when the same element is repeatedly used, there arises a problem in which it becomes difficult to selectively write information by using asteroid characteristics.
Accordingly, the TMR element is requested to have magnetic characteristics that can draw an ideal asteroid curve.
To draw an ideal asteroid curve, an R-H (resistance-magnetic field) loop obtained when magnetic characteristic of the TMR element are measured is free from noises such as a Barkhausen noise, a waveform should have an excellent rectangle property, the magnetization state should be stable and the coercive force Hc should be prevented from being fluctuated.
In order to read out information from the TMR element of the MRAM, information is read out from the TMR element by a difference current at a constant bias voltage and by a difference voltage at a constant bias current obtained in the condition in which the state of xe2x80x9c1xe2x80x9d, for example, is presented when the directions of the magnetic moments of one ferromagnetic material layer and the other ferromagnetic material layer sandwiching the tunnel barrier layer are anti-parallel and resistance values are high and in which the state of xe2x80x9c0xe2x80x9d is presented when the directions of the magnetic moments are parallel to each other.
Accordingly, when the fluctuations of the resistance values between the elements are the same, a higher TMR ratio (magnetoresistive changing ratio) is advantageous so that a memory which is high in speed, high in integration degree and which is low in error rate can be realized.
It is known that a TMR element having a fundamental structure of ferromagnetic material layer/tunnel barrier layer/ferromagnetic material layer has a bias voltage dependence of a TMR ratio so that the TMR ratio decreases as the bias voltage increases. In most cases, since it is known that the TMR ratio takes the maximum value of the read signal at a voltage (Vh) that is reduced by half due to the bias voltage dependence, smaller bias voltage dependence is effective for decreasing read errors.
Accordingly, the TMR element for use with the MRAM should satisfy the necessary conditions of the above-mentioned write characteristics and read characteristics at the same time.
However, when the material of the ferromagnetic material layer of the TMR element is selected, if alloy compositions by which spin polarizabilities shown by P1 and P2 in the equation (A) are increased are selected from the materials which are made of only ferromagnetic transition metal elements of Co, Fe, Ni, then there is a tendency that the coercive force Hc of the TMR element generally increases.
When a magnetization free layer (free layer), i.e. an information recording layer is made of a Co75Fe25 (atomic percent) alloy and the like, although the spin polarizability is large and a high TMR ratio greater than 40% can be maintained, the coercive force Hc also increases.
On the other hand, when the magnetization free layer is made of an Ni80Fe20 (atomic percent) alloy that is called a permalloy known as a soft magnetic material, although the coercive force Hc can be decreased, since the spin polarizability is low as compared with the above-mentioned Co75Fe25 (atomic percent) alloy, the TMR ratio is lowered to approximately 33%.
Further, when the magnetization free layer is made of a Co90Fe10 (atomic percent) alloy that has an intermediate characteristic of the above-mentioned alloys having the two compositions, although a TMR ratio of approximately 37% can be obtained and the coercive force Hc can be suppressed to approximately an intermediate value between the coercive force Hc of the above-mentioned Co75Fe25 (atomic percent) alloy and the coercive force Hc of the above-mentioned Ni80Fe20 (atomic percent) alloy, the rectangle property of the R-H loop (resistance-magnetic field curve) is not satisfactory and an asteroid characteristic for making writing become possible cannot be obtained. Furthermore, a problem arises in which the inverted magnetic field of the magnetization free layer is not stabilized at every element.
An object of the present invention is to provide a magnetoresistive effect element that can obtain excellent magnetic characteristics.
It is another object of the present invention to provide a magnetic memory device including this magnetoresistive effect element and which can obtain excellent write/read characteristics.
According to an aspect of the present invention, there is provided a magnetoresistive effect element in which a pair of ferromagnetic material layers is opposed to each other through an intermediate layer to obtain a magnetoresistive change by causing a current to flow in the direction perpendicular to the layer surface. One of the ferromagnetic material layers is a magnetization fixed layer and the other of the ferromagnetic material layer is a magnetization free layer, the magnetization free layer is made of a ferromagnetic material containing FeCoB or FeCoNiB and the magnetization free layer has a film thickness ranging from 2 nm to 8 nm.
According to other aspect of the present invention, there is provided a magnetic memory device comprising a magnetoresistive effect element designed in such a manner that a ferromagnetic tunnel junction sandwiching a tunnel barrier layer is formed between a pair of ferromagnetic material layers to cause a current to flow in the direction perpendicular to the layer surface and word lines and bit lines sandwiching the magnetoresistive effect element in the thickness direction. One of the ferromagnetic material layers is a magnetization fixed layer and the other ferromagnetic material layers is a magnetization free layer, the magnetization free layer is made of a ferromagnetic material containing FeCoB or FeCoNiB and the magnetization free layer has a film thickness ranging from 2 nm to 8 nm.
According to the arrangement of the magnetoresistive effect element of the present invention, since the magnetization free layer is made of the ferromagnetic material containing FeCoB or FeCoNiB and the magnetization free layer has the film thickness ranging from 2 nm to 8 nm, it becomes possible to increase a magnetoresistive changing ratio (magnetoresistive ratio) and to improve a rectangle property of a resistance-magnetic field curve and fluctuations of a coercive force.
According to the arrangement of the magnetic memory device of the present invention, since the magnetic memory device includes the magnetoresistive effect element and the word lines and the bit lines sandwiching the magnetoresistive effect element in the thickness direction and the magnetoresistive effect element has the arrangement of the magnetoresistive effect element of the present invention, magnetic characteristics of the magnetoresistive effect element, such as the magnetoresistive ratio, the rectangle property of the resistance-magnetic field curve and the fluctuations of the coercive force can be improved and hence errors caused when information is written in and read out from the magnetic memory device can be decreased.